A power MOSFET has a much simpler gate driving circuit than a bipolar transistor due to having a high input impedance. In addition, as a unipolar device, the power MOSFET is also advantageous in view of causing no time delay induced by accumulation or recombination of some minority carriers while the device is being turned off. Accordingly, use of the power MOSFET has been increasing in the fields of a switching mode power supply, a lamp ballast, a motor driving circuit, etc.
A double diffusion MOSFET (DMOSFET) structure using a planar diffusion technology has been generally applied to the power MOSFET. However, a trench gate-type MOSFET structure, so called a vertical gate-type MOSFET structure, in which trenches are formed by etching a semiconductor substrate by a predetermined depth and gates are formed in the trenches is being researched. The trench gate-type MOSFET is capable of implementing high integration and low source-drain on resistance (Rds(on)) by increasing the cell density per unit area while reducing a junction field-effect transistor (JFET) resistance among the devices.
As illustrated in example FIG. 1, the trench gate-type MOSFET is structured in a manner that low-density N-type epitaxial layer 2 is formed on and/or over high-density N-type substrate 1. P-type epitaxial layer 3 is formed on and/or over low-density N-type epitaxial layer 2. P-type epitaxial layer 3 and low-density N-type epitaxial layer 2 are etched to a predetermined depth, accordingly forming trench 4. Gate dielectric layer 5 is applied on and/or over sidewalls and bottom wall of trench 4. Gate electrode 6 is formed on and/or over gate dielectric layer 5 and to fill trench 4. High-density N-type dopant region 7 is formed on and/or over P-type epitaxial layer 3 around trench 4 formed with gate electrode 6. Accordingly, high-density N-type dopant region 7 becomes a source terminal of the MOSFET whereas high-density N-type substrate 1 becomes a drain terminal.
However, the above-described trench gate-type MOSFET has a variety of disadvantages. For instance, a bottom of substrate 1 should be in electric connection in order to apply signals to the drain terminal. Therefore, the trench gate-type MOSFET can be used only as a single device but cannot be integrated with a horizontal-type device. In addition, in a drain extended MOSFET (DEMOS) which is a horizontal-type high-voltage device, channels are formed horizontally. Accordingly, the area occupied by chips is increased to achieve great voltage and current capacity as desired.